Ive been wearing an assault vest since i left basic training, but unfortunatley due to a change in unit policy and SOPs Ive got to revert back to PLCE. Can anyone suggest a way of keeping the fooking belt sat right despite having five pouches on it? Its all adjusted and bungeed etc. but it always sags where the heavier pouches (water bottle and rations) are mounted. Its beginning to piss me off...

Get issued a belt that fits - they come in different sizes; get it tighter than tight - so it just can't sag; get the pouches closer together - you may find that more pouches=better fit as they form a natural curve around your waist; get bungeed and hippo-padded-up to the hilt if you want. GET USED TO IT! PLCE is probably one of the best bits of issued kit over the last 20 years ('58 pattern anyone?). And for conventional old-skool patrolling, getting prone etc, it is still the business.
As my old CSM used to say - "at the end of the day, there's only one cnut that wears a vest - and that's the cnut that drives a truck"

Sounds like you have been issued the wrong size belt to me, take the above advice and see if you can get a smaller one.
Bungees around the rear pouches will do the rest, failing that thread a utility strap through the bottom loops of the webbing pouches and ammo pouches and secure it at the front.